For the production of primary melt of iron, a shaft furnace configured as blast furnace is predominantly employed as main unit, while other methods merely have a corresponding share of only approximately 5%. This shaft furnace can operate according to the counterflow principle. Raw materials such as burden and coke are charged in the upper region of the shaft furnace of the furnace top and sink to the bottom in the shaft furnace. In a lower region of the furnace (blow mould level) a treatment gas (so-called blast air with a volume of 800-1 100 m3/tRE depending on the size of the furnace) is blown into the furnace through blow moulds. In the process, the hot blast, which usually is air heated in advance in blast preheaters to approximately 1 000 to 1300° C., reacts with the coke during which carbon monoxide is generated among other things. The carbon monoxide rises in the furnace and reduces the iron oxides and additional iron compounds contained in the burden.
In addition to this, substitute reduction agents with for example 100-200 kg/tRE (coal dust, oil, natural gas or plastic) are usually also blown into the furnace which promotes the generation of reduction gas.
In addition to the reduction of the iron ores the raw materials melt because of the heat generated by the chemical processes that occur in the shaft furnace. The gas distribution over the cross section of the shaft furnace however is irregular. For example in the centre of the shaft furnace the so-called “dead man” is formed while the relevant processes such as gasification (reaction of oxygen with coke or substitute reduction agents to form carbon monoxide and carbon dioxide) merely occurs in the so-called fluidised zone, which is a region in front of a blow mould, i.e. with respect to the cross section of the furnace is only located in a marginal region. The fluidised zone has a depth towards the furnace centre of approximately 1 m and a volume of approximately 1.5 m3. Usually a plurality of blow moulds are circumferentially arranged in the blow mould level in such a manner that the fluidised zone formed in front of each blow mould overlaps with the fluidised zones formed on the left and right or is located closely together, so that the active region is substantially provided by a circular region. The so-called “raceway” or fluidised zone forms during the operation of the shaft furnace.
Furthermore, the hot blast can usually be enriched with oxygen in order to intensify the processes (gasification in the fluidised zone, reduction of the iron ores) just described, which results in an increase of the performance of the shaft furnace. Here, the hot blast can for example be enriched with oxygen before feeding in, or pure oxygen can also be fed in separately, wherein for the separate feeding a so-called lance has to be provided, i.e. a pipe which extends for example within the blow mould, which itself is a pipe-like part, and terminates within the blow mould in the furnace. More preferably with modern blast furnaces, which are operated with low coke rate, the hot blast is suitably enriched with oxygen to a high degree. On the other hand the production costs are increased through the addition of oxygen so that the efficiency of a modern shaft furnace cannot simply be increased by corresponding addition of ever more increased oxygen concentration.
It is also known that the efficiency of a modern shaft furnace is correlated with the so-called through-gasification in the shaft furnace. Generally this means how well the gasification in the fluidised zone, the reduction of the iron ores and generally the draught of the gas phase prevailing in the shaft furnace operates from the blow mould level up to the top, where the so-called blast furnace gas is discharged. A sign of better through-gasification for example is the least loss of pressure possible in the furnace.
From WO 2007/054308 A2 it is known to operate a suitably configured shaft furnace in such a manner that the treatment gas introduced in the lower region of the blast furnace is pulsed at short time intervals. The pressure and/or the volumetric flow of the treatment gas are varied within a time span of less than 40 s, as a result of which the through-gasification of the shaft furnace and thus the efficiency of the shaft furnace are improved. Furthermore, the treatment gas before the introduction can be branched off with different pressures to the various blow moulds in the blow mould level in order to be able to set different peripheral conditions in different sectors of the blow mould level.
However there exists a continuous need for further improving the efficiency of the shaft furnace.